1. Field of the Invention
The present invention relates to a multicolor image forming apparatus and method, and in particular, to a multicolor image forming apparatus and method that can compensate for variations in laser beam scanning speed in a multicolor image forming mechanism that scans plural laser beams on a photoconductor and superimposes formed multiple images one upon another.
2. Description of the Related Art
In conventional multicolor image forming apparatuses using plural laser beams, distances of a photoconductor to laser scanner units for respective colors vary from color to color due to an installation error or a change in temperature. These variations, in turn, cause variations in scanning speeds of laser beams scanned on the photoconductor for image formation of respective colors.
It has been proposed to correct variations in laser beam scanning speed at each scanning time or unit-to-unit variation. For example, in Japanese patent unexamined publications 63-175817, 62-253115, 03-110512 and 61-162023, a technique such as one shown in FIG. 10 is proposed, which is basically provided with two light-detecting sensors at laser beam scanning start and end sides so that deviation of detection time interval will be fed back to pixel clock frequency.
Specifically, as shown in FIG. 10, a laser beam is scanned on a photoconductor 51 with a laser scanner unit constituted of a laser diode 58, a collimator lens 56, a polygon mirror 54 and an fxcex8 lens. Then, a scanning time is obtained by a leading end detector 52 and rear end detector 53. The scanning time is compared to a reference value by means of an operational circuit, not shown, to obtain deviation of the scanning time from the reference value, so that the oscillation frequency of an oscillator 55 from which a pixel clock is output will be controlled in accordance with the deviation.
Such a conventional technique, however, has two major drawbacks when being applied to a multicolor image forming apparatus.
The first drawback is that the mounting positions of the light-detecting sensors vary from color to color with respect to an area over which laser beams of respective colors are scanned on the photoconductor. The variations in the mounting position, in turn, cause displacement of images of respective colors formed on the photoconductor.
The second drawback is that the positions of respective laser scanner units relative to the photoconductor deviate in different angular directions from the design position due to an installation error or a change in temperature. Therefore, even if the mounting positions of the sensors for detecting scanning time of each laser beam happen to correspond with each other with respect to the area over which the laser beams of respective colors are scanned, correction of the pixel clock for each color based on the detected scanning time is not enough to scan each laser beam over the surface of the photoconductor at a constant speed. The variations in the scanning speed, in turn, causes displacement of images of respective colors.
These drawbacks make it difficult to avoid chromatic aberration in an image formed on the photoconductor.
It is an object of the present invention to provide a multicolor image forming apparatus and method that can prevent chromatic aberration caused by a deviation of difference in scanning speed among plural laser beams scanned on a photoconductor for image formation of respective colors.
In one form of the present invention, there is provided a multicolor image forming apparatus comprising a photoconductor to be driven; a plurality of laser beam scanning parts each of which scans one of a plurality of laser beams on the photoconductor in a direction substantially perpendicular to the traveling direction of the photoconductor; a multicolor image generating part for overlapping a plurality of monochromatic images of respective colors formed with the plurality of laser beams on the photoconductor to form a multicolor image; a plurality of scanning-speed measuring parts each of which measures the scanning speed of each of the plurality of laser beams scanned by each of the plurality of laser beam scanning parts along the surface of the photoconductor; image clock-frequency correcting parts each of which corrects each image clock frequency corresponding each of the plurality of laser beam scanning parts based on the measured value of the scanning speed of the laser beam scanned along the surface of the photoconductor; and a plurality of laser beam driving parts each of which drives a corresponding laser beam at the corrected image clock frequency.
According to the present invention, variations in the scanning speeds of the plurality of laser beams along the scanning surface of the photoconductor are measured to obtain a compensation value for each image clock frequency so that each of the plurality of laser beams will be driven at the corrected image clock frequency. Therefore, even if the distance of each laser scanner unit relative to the photoconductor varies due to an installation error or change in temperature, a latent image of desired image data can be formed in a desired position on the photoconductor. That is, the image formed by scanning the laser beams of respective colors and developed by each developing unit will be of high quality without color aberration.